1. Field of the Invention
This invention relates to chromatic aberration correcting elements for optical assemblies usable in television (video) cameras. As the photographic lens for a television camera is designed to be used in combination with a prism, (for example, a color separation prism of a specified medium of certain thickness on the image side of the photographic lens 0, when the prism in use has a different thickness or is composed of a different medium from the designed one, the chromatic aberration varies. Hence, the present invention relates to a chromatic aberration correcting element for use in such a photographic lens on the image side thereof to compensate for the variation of the chromatic aberration.
2. Description of the Related Art
The color television camera is conventionally equipped with a plurality of image pickup elements for 2 or 3 primary colors (R, G, B) to which polychromatic light from the photographic lens is conducted through a color separation (image separation) prism for the 2 or 3 primary colors. The photographic lens to be used in the color television camera is, therefore, designed in such a way that good correction of all image aberrations is established only when the color separation prism arranged on the image side of the lens is of a specified medium with a predetermined thickness.
For this reason, if a color separation prism which is different in medium and thickness from the designed one is used, large chromatic aberrations are produced, becoming a cause for lowering the image quality.
Suppose the color separation prism for which the aberration correction of a photographic lens has been attained is made of an optical glass, for example, of BK7 in the Schott catalogue, whose property is nd=1.51633, .upsilon.d=64.1 with a thickness of 65 mm. Then if a color separation prism comprised of a prism element of F5(nd=1.60342, .upsilon.d=38.0) 30 mm thick and another prism element of BK7 20 mm thick is attached instead, the inconsistency in material and prism length will result in the production of large chromatic aberrations, particularly longitudinal chromatic aberration or spherical aberration.
The spherical aberration can be corrected relatively easily by inserting a parallel flat glass to adjust the length of the optical path of the prism to the design value, as disclosed in Japanese Laid-Open Utility Model Application No. Sho 57-71313.
The longitudinal chromatic aberration is, on the other hand, difficult to correct when the parallel flat glass only is inserted. In a case where the BK7 correction lens is combined with the F5 prism through a 15 mm thick parallel flat glass of BK7, the blue image is focused as displaced from the position equivalent to the green image in a direction of going away from the lens, while the red image is focused as displaced relative to the green image toward the lens. By representation in terms of Fraunhofer lines, the g-line (436 nm) displaces 107 microns with respect to the d-line (588 nm), and the c-line (656 nm) displaces -22 microns with respect to the d-line.
The photographic lens for a television camera is designed in a form near to the so-called telecentric form with the exit pupil at infinity for the purpose of preventing the occurrence of color shading due to the change of the angle of incidence of light on the dichroic layers of the color separation prism. From this reason, even if the material of the prism in use is different from the designed one, it is only the longitudinal chromatic aberration that suffers a change, but the lateral chromatic aberration scarcely changes.
In the case of using a set of image pickup tubes as the image pickup means, because of their being axially movable, even if the photographic lens is left having some residual longitudinal chromatic aberration, the compensation for this could be accomplished by adjusting the positions of the image pickup tubes for the different colors individually to provide respective sharp focuses.
In recent years, however, an increasing number of color television cameras have employed a set of solid state image pickup elements in fixedly secured relation to the prism. In this case, because the possibility of adjusting the positions of the image pickup elements is no longer available, if a photographic lens of different compensation for the prism is attached, the longitudinal chromatic aberration collapses, giving rise to a problem of the so-called tracking error.
This phenomenon cannot be obviated even if the prism material is the same as the assumed material, since it takes place in a case where the standard wavelengths for the three primary colors R, G and B are different from the designed ones, or where the amount of primary chromatic aberration differs.
For example, a camera A is assumed to have a prism length of 65 mm with the R-channel and B-channel CCDs in different positions respectively by +5 microns and +30 microns relative to the G-channel CCD. Another camera B is assumed to have a prism length of 60 mm with the R-channel and B-channel CCDs in different positions each by +15 microns relative to the G-channel CCD. Suppose the photographic lens corrected for longitudinal chromatic aberration with respect to the camera A is attached to the camera B, then the use of only the 5 mm thick parallel flat glass does not suffice for matching the longitudinal chromatic aberration. For this reason, a tracking error occurs.